1. Field of the Invention
This invention relates to mixers in general and more particularly to a triple balanced mixer that provides high isolation in a small package size.
2. Description of the Prior Art
A mixer circuit converts a radio frequency (RF) signal to an intermediate frequency (IF) signal which is the difference of the RF and a local oscillator (LO) signal. The IF frequency is obtained by multiplying the RF signal with the local oscillator (LO) signal. The difference or IF frequency is a result of the non-linearity of the mixer. Along with the IF frequency, the mixer typically generates inter-modulation products due to the non-linearity response.
Isolation is a measure of the circuit balance within the mixer. When the isolation is high, the amount of leakage or feed through between the mixer ports will be very small. Typically, isolation decreases as frequency increases due to the unbalance in the mixer circuit. Isolation can be measured as LO-RF isolation which is the amount the LO drive level is attenuated when it is measured at the RF port. LO to IF isolation is the amount the LO drive level is attenuated when it is measured at the IF port.
Mixers are typically designed with one of three topologies: single ended, balanced, and double balanced. The double balanced mixers are capable of isolating both the RF signal and the local oscillator LO voltages from the output and thus allow overlap of the RF and IF frequency bandwidths. Several prior art mixer circuits are well known. Double balanced and triple balanced mixers have been described in the prior art. Double balanced mixers use a diode ring and two baluns. One of the baluns separates the local oscillator and intermediate frequency signal or the RF and intermediate frequency signal. An extension of this concept is the use of two ring diodes and three separate baluns, one each for RF, LO and IF signals. This approach has the advantage of having very good isolation and low inter-modulation products but requires a 3 dB higher RF power level.
For frequencies below 5 GHz, ferrite transformers are used for the baluns. The ferrite transformers are large and require excessive circuit board space. In addition the ferrite transformers are difficult to assemble to a circuit board requiring a lengthy manual assembly process by a skilled operator. This adds undesirable cost to the product and is difficult for large scale manufacturing.
Baluns can also be implemented by transmissions lines such as a suspended microstrip or a microstrip formed on a printed circuit board. Transmission line baluns have the disadvantage that they require large amounts of space for the transmission lines to be run along the printed circuit board.
While various mixers have been used, they have suffered from being difficult to assemble, expensive to produce and requiring excessive circuit board space. A current unmet need exists for a triple balanced mixer that is compact and easily assembled.